What limit my connections?

I use netTcpBinding:
      <netTcpBinding>
        <binding name="NewBinding0" maxBufferPoolSize="52428800" maxBufferSize="6553600"
          maxConnections="1000" maxReceivedMessageSize="6553600" />
      </netTcpBinding>
and my behavior is:
        <behavior name="ConcurrentBehavior">
          <serviceThrottling maxConcurrentCalls="1000" maxConcurrentSessions="1000"
            maxConcurrentInstances="1000" />
        </behavior>

This is my test code:/Files/bihpgh20/What limit my connections/ConcurrentTest.rar

I think with this config I can create 1000 client proxies conntceting to service currently, but unfornately, I got an EndpointNotFoundException after receive several responses from service(Test_1); If I send 10 requests, and then send 15 requests, and then send 20 requests, and so on... It's OK(Test_3).

Wait for a moment, My Test_3 sometimes still give me an EndpointNotFoundException.

I do not know what is the real reason. Please help me?